When power supply voltage is first connected to a circuit, the initial state of the circuit is unknown. Low voltage detect circuits are used to detect the voltage at the power supply terminal, and reset the initial state of the circuit once that voltage crosses a predetermined turn-on point.
One approach to the design of low voltage detect circuits relies on the turn-on voltage of a transistor (Vbe in the case of bipolar, or Vt in the case of MOS transistors) to define the turn-on point. These circuits are therefore sensitive to the absolute value of a threshold voltage (or Vbe) as well as the temperature coefficient of the threshold voltage (or Vbe). To improve their accuracy, the variation in the absolute value is corrected at die sort using trimming techniques. However, temperature sensitivity of these circuits will still cause variations in the turn-on point. This can be a problem, especially when there is a narrow window between the minimum power supply voltage the circuit might receive and the maximum power supply voltage the circuit might require for proper operation.
Another approach to the design of low voltage detect circuits uses a capacitor to hold a node voltage until the power supply voltage is high enough to turn on a switch that discharges the capacitor. This approach, however, requires a fast power supply voltage rise time. In the case of a slow rising power supply voltage, leakage could reduce the charge on the capacitor to low enough levels to signal power-on, before the power supply voltage has reached the turn-on point.
Therefore, existing low voltage detect circuits do not satisfy the requirements of a fast and accurate low voltage detect signal that has very little variation over process and temperature.